The invention relates to identifying the informational significance of binary bits that are transmitted acoustically through a drill string by frequency-shift-keyed modulation. Systems have been previously developed for acoustically telemetering signals in boreholes. Typical of these systems are those disclosed in applicant's U.S. Pat. Nos. 3,889,228 and 3,930,220. Circuits have also been developed for detecting the telemetered signals in the presence of the high background noise associated with drilling boreholes. An example is found in Applicant's U.S. Pat. No. 4,019,148.
A convenient way of encoding information for transmission through a drill string in an acoustical telemetry system is frequency-shift-keyed modulation, although a single frequency system could be used. The information, which generally concerns borehole parameters such as inclination, pressure, or temperature, are converted from analog form to digitally coded words according to well-known digital methods. These digitally coded words are then used to modulate the frequency-shift-keyed (FSK) system. The FSK system represents digital data by shifting between two frequencies that are relatively close together but distinguishable. One frequency is used to represent a binary "0", and the other to represent a binary "1", and by shifting between the two frequencies in the proper sequence binary words can be represented. The encoded FSK signals can then be used to drive an electro-acoustical transducer, or other suitable device, which induces the desired signals into the drill string in the form of acoustical signals.
These acoustical signals are conducted along the drill string, generally from the bottom of the drill string to the surface. Attenuation may be compensated for by placing repeaters at regular intervals along the drill string.
Both of the surface and in each repeater the FSK signal must be detected, distinguished from the background noise and the informational significance of the components thereof identified.